This invention relates to heat storage ponds such as solar collectors, and to power plants utilizing the same.
A body of water provided with a surface mechanism that suppresses convection currents acts as an insulator for heat accumulated in the body of water. When the heat is produced at the absorption of solar radiation, the body of water is termed a solar pond. In the absence of such a mechanism, heat added to the lower level of the body of water is transferred by convection currents to the upper level where transfer to the environment takes place by radiation and/or by conduction at the liquid-air interface. As a consequence, the vertical temperature gradient in a body of water, in which convection currents are not suppressed, is too small to be of practical value in operating a power plant.
A conventional mechanism for suppressing convection currents is achieved by establishing and maintaining a halocline above a heat storage zone in a body water. The halocline is a layer of water containing a salt concentration whose density gradient is in the direction of gravity. The halocline permits a corresponding temperature gradient to be established in the halocline as solar radiation is absorbed thereby enabling the halocline to act as a thermal barrier or insulation layer protecting the heat storage zone therebelow against conductive heat loss to the environment above the halocline. As a consequence, the water near the surface remains relatively cool as the water near the darkened bottom is heated. In such case, the resultant temperature differential is sufficiently large to allow operation of a heat engine that converts some of the absorbed solar heat to work.
The basic problem with this approach is the time-wise instability of the density gradient which can be maintained over long periods of time only by cycling different concentrations of solution between various levels of the pond. Furthermore, great care must be exerted in extracting heat from the pond so as not to disturb the density gradient. Much effort has been devoted to equipment for stabilizing density gradients in solar ponds, but the equipment of the prior art remains complicated with a result that long term reliability is relatively low and the solar pond concept has not been widely adopted for collecting solar heat. Therefore, it is an object of the present invention to provide a new and improved heat storage pond, and to provide a power plant utilizing the same, wherein the deficiencies of prior art solar ponds are overcome or substantially diminished.
In accordance with the present invention, there is provided a heat storage pond comprising a heat storage liquid at a temperature higher than an ambient medium, and an insulating layer that floats on the liquid and is thick enough to thermally insulate the latter against significant conductive heat loss to the ambient medium above the insulating layer. The insulating layer can be non-liqud, and preferably is a gel that is transparent to visible light and near infra-red rays from the sun which are absorbed by the heat storage liquid which can be water or a salt hydrate.
A suitable aqueous gel includes cross-linked polyacrylamide, while suitable non-aqueous gels can be formed from hydrocarbon liquids such as kerosene, paraffin oil or silicone oil. Gels formed from such hydrocarbon liquids include cross-linked polyisobutilene.
In one embodiment of the invention, the ambient medium above the gel is a layer of water which transmits most of the visible light and near infra-red components of the solar radiation. The layer of water thus remains relatively cool as compared to the temperature of the heat storage liquid which is heated by the absorption of solar radiation.
An aqueous gel may be produced in situ by adding acrylamide to a segregated layer of water above the heat storage liquid together with a polymerizing agent and an agent that produces cross-linking. Alternatively, a non-aqueous gel can be produced by polymerizing isobutilene in a kerosene base in the presence of an agent that produces cross-linking. This latter gel has the advantage of having a heat conductivity that is approximately 50% of the heat conductivity of a water based gel, and as a consequence, better insulates the heat storage liquid. Finally, the heat storage liquid can be contained in a depression in the earth, or can be contained in a tank. Alternatively, the heat storage liquid can be a body of water, natural or artificial.
The invention also consists in a solar power plant utilizing a heat storage pond in accordance with the present invention including a heat engine for extracting heat from the heat storage liquid. Preferably, the heat engine rejects heat into the layer of water above the gel. In the preferred embodiment of the invention, the working fluid of the heat engine is different from the heat storage liquid. In such case, the heat engine includes a first heat exchanger in thermal contact with the heat storage liquid for heating the working fluid, an energy converter responsive to the passage of heated working fluid for converting some of the heat in the working fluid into work, and a second heat exchanger in thermal contact with the layer of water over the gel for condensing the working fluid after it passes through the converter. Preferably, the working fluid is an organic fluid and the heat engine is a turbine.